Add toPluginDocument() convenience method that takes a reference in argument

Source/core/html/HTMLEmbedElement.cpp

Index: Source/core/html/HTMLEmbedElement.cpp
diff --git a/Source/core/html/HTMLEmbedElement.cpp b/Source/core/html/HTMLEmbedElement.cpp
index 9e45aefe908409381f2a161c2b031d62fe5c92cf..99313a7117205bfd25e0b9987357fca9534c8e0b 100644
--- a/Source/core/html/HTMLEmbedElement.cpp
+++ b/Source/core/html/HTMLEmbedElement.cpp
@@ -162,7 +162,7 @@ void HTMLEmbedElement::updateWidget(PluginCreationOption pluginCreationOption)
             // Plugins inside plugin documents load differently than other plugins. By the time
             // we are here in a plugin document, the load of the plugin (which is the plugin document's
             // main resource) has already started. We need to explicitly cancel the main resource load here.
-            toPluginDocument(&document())->cancelManualPluginLoad();
+            toPluginDocument(document()).cancelManualPluginLoad();
         }
         return;
     }